1. Field of the Invention
The present invention relates to control of a hydraulic stepless transmission (hereinafter, referred to as HST) equipped on a vehicle.
2. Background Art
Conventionally, there is a well-known HST as a transmission for a vehicle, especially for a working vehicle such as a tractor. The International Publication WO00/25041 and the Japanese Patent Laid Open Gazette 2001-108061 indicate a hydro mechanical transmission (hereinafter, referred to as HMT) having a differential mechanism driven by rotation difference between input and output of the HST (for example, a planetary gear mechanism) as the transmission for a working vehicle. With regard to this transmission, at the time of traveling forward in low speed and traveling backward, driving wheels are driven by output of the HST. At the time of traveling forward in middle or high speed, the driving wheels are driven by output of the differential mechanism so as to obtain high output rotation speed with reducing loss of output of the HST. A state of the transmission set as the former is referred to as a HST mode, and a state of the transmission set as the later is referred to as a HMT mode. Specifically, a clutch device is operated to select either inputting the output of the differential mechanism to a driving shaft for transferring output to the driving wheels or inputting the output of the HST directly to the driving shaft without using the differential mechanism.
Furthermore, with regard to these cited technologies, output/input speed ratio of the HST (hereinafter, referred to as “HST speed change ratio”) is controlled electronically. Namely, a hydraulic servomechanism or the like is used as an actuator for controlling the angle of a movable swash plate of the HST. Fundamentally, the actuator is controlled based on electric detection signals concerning operation of speed change means (a speed change pedal, a speed change lever or the like). The clutch device for switching between the HST mode and the HMT mode also electronically controlled in association with the control of the HST speed change ratio.
Moreover, with regard to both the cited documents WO00/25041 and 2001-108061, the output of the actuator is controlled with modifying an original target output value of the actuator based on the electric detection signals concerning operation of the speed change means corresponding to various conditions. For example, the International Publication WO00/25041 indicates compensational control of the HST output in consideration of engine load. The transmission published in the Japanese Patent Laid Open Gazette 2001-108061 is constructed so as to maintain setting speed by interlocking the swash plate actuator of the HST with an electronic governor disposed with the engine. Otherwise, such the transmission is mounted in a working vehicle such as a tractor, and the angle of the movable swash plate of a hydraulic pump disposed in the HST (HST swash plate angle) is controlled electronically by a controller, whereby the speed is controlled with considering rotation of a PTO shaft for driving a working machine (a rotary working machine, etc.).
However, there is no conventional transmission which controls output of the HST with considering load applied to the HST itself. The load applied to the HST appears with an aspect of raising oil pressure and temperature in a circuit. When oil pressure is raised, volume efficiency is decreased by leak and compression of oil according to characteristic of the HST, whereby rotation speed of an output shaft is changed even if the angle of the movable swash plate of the hydraulic pump is fixed. Namely, the speed change ratio of the HST is changed. Whether it is set in the HMT mode or the HST mode, rotation speed of the drive wheels depends on the HST speed change ratio. Therefore, unless the load applied to the HST is considered in the controlling, desired vehicle speed, speeding up and slowdown of the vehicle may not be obtained.
With regard to the control of the transmission with considering the load applied to the HST, a detection mean for the load applied to the HST must be considered firstly. Providing a detection mean of rising degree of oil pressure in the HST is handicapped in complexity of structure, cost and compactness of the HST, thereby being inexpedient.
With regard to feedback control of the HST swash plate angle controlling actuator with considering the load applied to the HST, the HST speed change ratio is set more highly, whereby action of the actuator becomes larger. With regard to the HST mode, the HST speed change ratio is increased by increasing the swash plate angle corresponding to the increase of the HST output rotation speed at the time of traveling forward. On the contrary, with regard to the HMT mode, the swash plate angle (the HST speed change ratio) is decreased against the increase of vehicle speed. Therefore, with regard to the speeding up operation requiring switching between the HST mode and the HMT mode, the change of the HST speed change ratio at the time of switching the mode becomes larger, whereby shock attendant upon the switch of the mode becomes larger.
Conventionally, with regard to the electronic control of HST swash plate angle, a dead band -3- for zeroing the swash plate angle is provided against command current, that is, at which the HST is kept in neutral is provided so as not to fail to keep a neutral position of the swash plate against few errors between the main speed changing operation and the HST swash plate angle. However, under the HMT mode, the timing that the HST is tuned into the neutral state is not limited to the timing for zeroing the vehicle speed, and the neutral state of the HST is equivalent to one of setting speeds. In this case, for example, when a operator does speed up operation, the dead band causes the vehicle to be kept in a certain speed during a fixed period far from being speeded up when the vehicle reaches the certain speed, whereby smooth speeding up is not obtained.
In many cases, individual electromagnetic valves for controlling the actuator have slightly different dead bands, whereby individual differences of action at the time of speed change may occur among vehicles.
Incidentally, a tractor equipped with a rotary cultivator or the like is an example of the vehicle driven by such the HST. Each field where the tractor performs rotary cultivation is not uniform in conditions such as hardness and viscosity, so that, while the vehicle advancing, the rotary cultivator passes one area requiring careful cultivation and another area which can be cultivated comparatively easily. If the field has such a spot requiring the careful cultivation, an operator decreases cultivation speed for cultivating the spot.
When the vehicle turns at a headland, the operator decreases rotation speed of the engine in consideration of two points, that is, safety at the time of decreasing rotation speed of the rotary working machine and prevention of breakage of a coupling member (universal joint) between the PTO shaft and the rotary working machine.
However, the operation of changing cultivation speed while checking the field condition is a burden to the operator and disturbs his/her concentration on steering operation.
Conventionally, the two points to be considered at the time of turning are dealt with by reducing rotation speed of the engine. Accordingly, cultivation speed can't be increased at the time of turning, thereby taking time.
Considering that the rotation speed and torque of the engine mounted in the tractor determine the fuel consumption efficiency of the engine, it is desirable to control the rotation speed and the torque of the engine so as to optimize the fuel consumption efficiency. However, the cultivation work with the tractor having the rotary cultivator may require the tractor to drive at a fixed speed (to keep a constant cultivation speed) so as to finish work within a predetermined time.
Further, the conventional working vehicle such as the tractor is parked by applying a parking brake after its traveling transmission is turned into neutral.
However, the conventional working vehicle such as the tractor may be stopped by actuating the parking brake without turning the traveling transmission into neutral, or may be started while the parking brake is still applied, thereby causing abrasion of a brake disk. When an operation part, such as a switch, for turning the vehicle into neutral is required, a parts count is increased.